As water and steam inside the boiler steam drum are both in the forms of high temperature and high pressure, and the changes in work conditions are complicated, the measurement of its liquid level has always been a difficult problem. The principle of measurement with balance container for most liquid level gauges of boiler steam drums used for adjustment and protection is adopted at present, while the measurement of steam drum with balance container is affected by work conditions, the measurement has often been inaccurate and failed for use during boiler startup and boiler shutdown, and additionally it may possibly give rise to failure of normal measurement when anomalous work condition is confronted during boiler operation, which has grievously affected the safe operation of boiler. While as only discontinuous measurement of liquid level can be applied to liquid level with electrical contact level gauges during boiler startup and shutdown or under anomalous work condition, namely the displayed liquid level can only be a range, therefore it cannot be used for automatic regulation of water level, and at the same time for the restriction of reliability, it is generally neither used for protection of liquid level.
The pressure of boiler steam drum varies from 2 Mpa to 20 Mpa in general according to the size of unit, and the corresponding temperatures of saturated water and steam vary from 212° C. to 365° C. The dielectric constants of water and steam at normal temperature are 80 and 1, respectively, the dielectric constants of water and steam at 365° C. are 9.94 and 2.65, respectively, the change in dielectric constant of water is approximately 10 times, while the conventional capacitive level gauge can only be used on the occasions when the dielectric constant of medium measured is relative stable, and therefore, it can neither be used for liquid level measurement of boiler steam drums.